Yellowing of a Clearcoat
We apply a clearcoat powder material over a steel substrate. We sometimes have problems with yellowing of the coating. What can cause this problem?
The amount of film applied or the cure oven can cause the yellowing of a clearcoat powder. First, check that the film meets the manufacturer's specifications. If the film thickness is correct, the problem could be related to overcure or oven fouling. Run a temperature recorder and make sure that the time and temperature are within the recommended range. If the film thickness and cure cycle are accurate, the final possible cause is inadequate exhaust volumes. If the exhaust rate is too low, the oven will build up with gases that can cause the discoloration of the coating.
Recoating Appearance Problems
We occasionally need to apply a second coat of powder due to minor defects such as light coating or dirt. When we apply the second coat, we often have problems with poor appearance and a rough, textured film, especially around the edges of the part. Can you offer any advice on how to recoat a part without causing these appearance problems?
First, work on the causes of the defects. Light coats and other appearance defects are frequently related to a lack of process controls. Accurate and consistent gun settings, a controlled environment, good maintenance and understanding electrostatic application can help reduce defects.
When it is necessary to recoat, it is important to make a few simple adjustments to the application process for good results. Parts that are recoated must have 100% coverage to avoid a "dry-spray" look. Parts that have been coated once will be more resistant to electrostatic attraction. The ionization from the gun will rapidly build on the surface because there is no simple path to earth ground.
As the powder begins to build on the surface, the excess ionization will interfere with electrostatic attraction and cause irregularities and texture in the film. Powder will have difficulty building near edges, and "stars" may appear in the coated surface. This is referred to as back-ionization. To overcome back-ionization, the current levels must be limited to avoid excess ions from building on the surface.
Typical adjustments for recoating include a reduction of current levels, a slightly farther gun-to-target distance and a slight increase in flow rates. Of course, this is easier if the recoats are segregated and coated in batches. Often the recoat parts are on the line with raw parts. In this case, it may be necessary to use one setting that works well for both.
White Specks on Wheel Rims
My automotive dealer had all four of my wheels refinished due to weather damage over the winter. The company that refinished these wheels used a black powder coating and then a clear powder coat over the top. After thoroughly inspecting the wheels two days after I picked up the car, I noticed white specks throughout all four wheels. My theory is that the clearcoat had some type of cross contamination in the coating process. The company that did the process is trying to tell me that the wheels are contaminated but not from the coating process. At this point, they are going to attempt to finely sand down some of the clearcoat and reapply. I'm afraid this isn't going to solve the problem 100%. Is it possible for you to conclude with my theory that the contamination is in the coating process, or can you point me to someone who can?
It is not possible to say with 100% certainty where the white specks came from. However, contamination on the wheel is highly unlikely. The wheel should have been thoroughly cleaned before the powder was applied. White specks would have been covered by the black powder. Sanding and recoating with clear powder will not cover up any remaining visible white specks. The chances are good that your conclusion that the powder was contaminated is correct.
